Does laser diode irradiation improve the degree of conversion of simplified dentin bonding systems?

Abstract Simplified dentin-bonding systems are clinically employed for most adhesive procedures, and they are prone to hydrolytic degradation. Objective This study aimed to investigate the effect of laser diode irradiation on the degree of conversion (DC), water sorption (WS), and water solubility (WSB) of these bonding systems in an attempt to improve their physico-mechanical resistance. Material and Methods Two bonding agents were tested: a two-step total-etch system [Adper™ Single Bond 2, 3M ESPE (SB)] and a universal system [Adper™ Single Bond Universal, 3M ESPE (SU)]. Square-shaped specimens were prepared and assigned into 4 groups (n=5): SB and SU (control groups – no laser irradiation) and SB-L and SU-L [SB and SU laser (L) – irradiated groups]. DC was assessed using Fourier transform infrared spectroscopy with attenuated total reflectance. Additional uncured resin samples (≈3.0 µL, n=5) of each adhesive were also scanned for final DC calculation. For WS/WSB tests, similar specimens (n=10) were prepared and measured by monitoring the mass changes after dehydration/water storage cycles. For both tests, adhesive fluids were dropped into standardized Teflon molds (6.0×6.0×1.0 mm), irradiated with a 970-nm laser diode, and then polymerized with an LED-curing unit (1 W/cm2). Results Laser irradiation immediately before photopolymerization increased the DC (%) of the tested adhesives: SB-L>SB>SU-L>SU. For WS/WSB (μg/mm3), only the dentin bonding system (DBS) was a significant factor (p<0.05): SB>SU. Conclusion Irradiation with a laser diode improved the degree of conversion of all tested simplified dentin bonding systems, with no impact on water sorption and solubility.


Introduction
Previous studies have indicated that an increase in temperature could enhance the mechanical properties of dentin bonding systems 7,25 . Despite these advantages, some concerns limit their clinical indications, since the heat could damage pulp tissue, thereby compromising dental vitality 14,27 .
In this scenario, the association of lasers with dentin bonding systems has been investigated to achieve a more resistant hybrid layer. Gonçalves, et al. 13 (1999) assessed Nd:YLF laser irradiation over a three-step, etch-and-rinse system prior to curing, which promoted an increase in dentin bond strength values. These authors attributed this performance to the creation of a new substrate composed of recrystallized hydroxyapatite after being melted in the presence of resin monomers, resulting in a substrate that is physically more resistant. With the same purpose, Maenosono, et al. 17 (2015) also showed that the use of a laser diode improved bond strength when associated with simplified dentin bonding systems (SDBSs). In addition to the role of the laser's interaction with dentin, the authors also emphasized the evaporation of solvents as an advantage of laser use, reducing the bond's susceptibility to degradation over time 16 .
Both lasers presented similar wavelengths (1047 nm for Nd:YLF, and 970 nm for laser diode), which partially explains the successful performance in these studies. As the laser diode presents additional interesting characteristics, such as versatility, smaller dimensions, and lower cost, it appears to be the more attractive option 17 . Despite these favorable performances by bondstrength tests, it is important to understand how lasers affect the polymerization process of SDBSs.
Any strategies that could reduce their susceptibility to hydrolytic degradation are desirable, as most of their failure is attributed to this limitation 24 . Water is an essential component for the hybridization process, as it produces expansion of the collagen brils, thereby allowing the penetration of dental adhesives into demineralized dentin 21,23 . However, residual water in the hybrid layer leads to hydrolytic degradation, impairing the polymerization of the dental adhesives and increasing their solubilization 3 .
Therefore, this study aimed to analyze the in uence of laser diode irradiation on the degree of conversion (DC) and water sorption/solubility (WS/ WSB) of uncured SDBSs. The null hypotheses were as follows: (1)  The quantitative response variables were DC (%), WS ( g/mm ), and WSB ( g/mm ).
The materials used are described in Figure 1.

Sample preparation
This study was performed in line with ISO 4049:2000 standard speci cations, except for the specimen dimensions. Square-shaped Te on molds (6.0×6.0×1.0 mm) were used to prepare the samples.
The SDBSs were dropped to ll them. The specimens were air-dried smoothly for 20 s, from a distance of 10 cm, to help solvent evaporation 6,9,15 .
In the laser groups (L), the SDBSs were irradiated with a laser diode (Siro LASER, Sirona Dental Systems,  Care was taken to place the tip perpendicularly to the sample surface, covering the entire specimen surface.

DC
In general, when attenuated total reflectance (ATR)-Fourier transform infrared spectroscopy (FTIR) is used to calculate the DC, each SDBS is commonly dropped on the ATR crystal, and one run is performed.
Subsequently, the same sample is polymerized, and the measure is taken again. However, it was necessary to standardize laser irradiation in this study, which implied the need for two different specimens for each condition. Additionally, square-shaped Te on molds were used to prepare the specimens.

Discussion
Preheating was performed before curing the resinbased dental materials. Heating these materials favors the increase of radical mobility 7 , promoting higher DC and lower WS/WSB1m 1,4,25 . Therefore, laser irradiation has also been indicated to heat the adhesive system and improve these properties.
The rst null hypothesis tested in this study was rejected, as laser irradiation provided higher DC for all SDBSs ( It promotes solvent evaporation and optimizes the DC.
Moreover, the increase in temperature also increases vapor pressure, improving its evaporation. However, oxygen can inhibit the polymerization of resin-based material, which was not considered in this study 8,15 .
Based on the literature, the performance of the laser diode on the SDBSs suggests that this could be an interesting option, as it favors the improvement of the DC in safe and more realistic clinical conditions. Batista, et al. 2 (2015) reported that the use of an Nd:YAG laser on the uncured adhesive promoted a greater degree of evaporation of solvents, and this was directly in uenced by their physicochemical properties.
As the tested bonding systems contain solvents, the use of laser could promote their evaporation

Conclusion
Considering the limitations of this study, we can conclude that laser diode irradiation improved the DC of the tested SDBSs, with no impact on WS and WSB.

Con ict of interest
The authors declare no con ict of interest with any companies whose products were involved in this study.